11 Semiconductor diodes
Exercise 36 Further problems on semiconductor materials and p-n junctions
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108 Electrical Circuit Theory and Technology
Now try the following exercise.
Exercise 36 Further problems on semiconductor
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Semiconductor diodes 109
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power supplies, zener diodes for use as voltage reference sources, light emitting diodes, and varactor diodes. Fig- ure 11.14 shows the symbols used to represent diodes in electronic circuit diagrams, where ‘a’ is the anode and ‘k’
the cathode.
k
a (a) Signal or rectifier diode
(b) Zener diode (c) Silicon controlled rectifier (thyristor) k
a
k
a
+
(d) Bridge rectifier (e) Triac mt2 mt1
g
g
k
a (f) Light emitting diode
k
(g) Photodiode a
(h) Varactor diode k
a
Figure 11.14
Table 11.1 Characteristics of some typical signal and rectifier diodes
Device Material Max repetitive Max forward Max reverse Application
code reverse voltage current current
(VRRM) (IF(max)) (IR(max))
1N4148 Silicon 100 V 75 mA 25 nA General purpose
1N914 Silicon 100 V 75 mA 25 nA General purpose
AA113 Germanium 60 V 10 mA 200µA RF detector
OA47 Germanium 25 V 110 mA 100µA Signal detector
OA91 Germanium 115 V 50 mA 275µA General purpose
1N4001 Silicon 50 V 1 A 10µA Low voltage rectifier
1N5404 Silicon 400 V 3 A 10µA High voltage rectifier
BY127 Silicon 1250 V 1 A 10µA High voltage rectifier
11.7 Characteristics and maximum ratings Signal diodes require consistent forward characteristics with low forward voltage drop. Rectifier diodes need to be able to cope with high values of reverse voltage and large values of forward current, and consistency of char- acteristics is of secondary importance in such applications.
Table 11.1 summarizes the characteristics of some com- mon semiconductor diodes. It is worth noting that diodes are limited by the amount of forward current and reverse voltage they can withstand. This limit is based on the physical size and construction of the diode.
A typical general-purpose diode may be specified as having a forward threshold voltage of 0.6 V and a reverse breakdown voltage of 200 V. If the latter is exceeded, the diode may suffer irreversible damage. Typical val- ues of maximum repetitive reverse voltage (VRRM) or peak inverse voltage (PIV) range from about 50 V to over 500 V. The reverse voltage may be increased until the maximum reverse voltage for which the diode is rated is reached. If this voltage is exceeded the junc- tion may break down and the diode may suffer permanent damage.
11.8 Rectification
The process of obtaining unidirectional currents and volt- ages from alternating currents and voltages is called rectification. Semiconductor diodes are commonly used to convert alternating current (a.c.) to direct current (d.c.), in which case they are referred to as rectifiers. The sim- plest form of rectifier circuit makes use of a single diode and, since it operates on only either positive or negative halfcycles of the supply, it is known as a half-wave rec- tifier. Four diodes are connected as a bridge rectifier — see Figure 11.14(d) — and are often used as a full-wave rectifier. Note that in both cases, automatic switching of the current is carried out by the diode(s). For methods of
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half-wave and full-wave rectification, see Section 14.7, page 166.
11.9 Zener diodes
Zener diodes are heavily doped silicon diodes that, unlike normal diodes, exhibit an abrupt reverse breakdown at relatively low voltages (typically less than 6 V). A similar effect, called avalanche breakdown, occurs in less heavily doped diodes. These avalanche diodes also exhibit a rapid breakdown with negligible current flowing below the avalanche voltage and a relatively large cur- rent flowing once the avalanche voltage has been reached.
For avalanche diodes, this breakdown voltage usually occurs at voltages above 6 V. In practice, however, both types of diode are referred to as Zener diodes. The symbol for a Zener diode is shown in Figure 11.14(b) whilst a typical Zener diode characteristic is shown in Figure 11.15.
Forward current (mA)
Reverse voltage (V)
Reverse current (mA)
Forward voltage (V)
−20 −15 −10 −5 20
10
0 1 2 3
−10
−20
−30
−40
Figure 11.15
Whereas reverse breakdown is a highly undesirable effect in circuits that use conventional diodes, it can be extremely useful in the case of Zener diodes where the breakdown voltage is precisely known. When a diode is undergoing reverse breakdown and provided its maximum
ratings are not exceeded the voltage appearing across it will remain substantially constant (equal to the nominal Zener voltage) regardless of the current flowing. This property makes the Zener diode ideal for use as a voltage regulator.
Zener diodes are available in various families (accord- ing to their general characteristics, encapsulations and power ratings) with reverse breakdown (Zener) voltages in the range 2.4 V to 91 V.
Problem 7. The characteristic of a Zener diode is shown in Figure 11.16. Use the characteristic to deter- mine (a) the current flowing in the diode when a reverse voltage of 30 V is applied, (b) the voltage dropped across the diode when a reverse current of 5 mA is flowing in it, (c) the voltage rating for the Zener diode, and (d) the power dissipated in the Zener diode when a reverse voltage of 30 V appears across it.
Forward current (mA)
Reverse voltage (V)
Forward voltage (V)
Reverse current (mA)
−40 −30V −27.5V −20
−10
−5mA
−32.5 mA
−10 0 10 20
1 2 3
−20
−30
−40
Figure 11.16
(a) When V= −30 V, the current flowing in the diode, I= −32.5 mA
(b) When I= −5 mA, the voltage dropped across the diode, V= −27.5 V
(c) The characteristic shows the onset of Zener action at 27 V; this would suggest a Zener voltage rating of 27 V
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(d) Power, P=V×I, from which, power dissipated when the reverse voltage is 30 V,
P=30×(32.5×10−3)=0.975 W=975 mW
11.10 Silicon controlled rectifiers
Silicon controlled rectifiers (or thyristors) are three- terminal devices which can be used for switching and a.c. power control. Silicon controlled rectifiers can switch very rapidly from conducting to a non-conducting state. In the off state, the silicon controlled rectifier exhibits neg- ligible leakage current, while in the on state the device exhibits very low resistance. This results in very little power loss within the silicon controlled rectifier even when appreciable power levels are being controlled.
Once switched into the conducting state, the silicon controlled rectifier will remain conducting (i.e. it is latched in the on state) until the forward current is removed from the device. In d.c. applications this necessitates the interruption (or disconnection) of the supply before the device can be reset into its non-conducting state.
Where the device is used with an alternating supply, the device will automatically become reset whenever the main supply reverses. The device can then be triggered on the next half cycle having correct polarity to permit conduction.
Like their conventional silicon diode counterparts, sil- icon controlled rectifiers have anode and cathode connec- tions; control is applied by means of a gate terminal, g.
The symbol for a silicon controlled rectifier is shown in Figure 11.14(c).
In normal use, a silicon controlled rectifier (SCR) is triggered into the conducting (on) state by means of the application of a current pulse to the gate terminal — see Figure 11.17. The effective triggering of a silicon con- trolled rectifier requires a gate trigger pulse having a fast rise time derived from a low-resistance source. Trigger- ing can become erratic when insufficient gate current is available or when the gate current changes slowly.
Controlled load, RL
RG
Gate trigger SCR pulse
AC or DC supply
Figure 11.17
A typical silicon controlled rectifier for mains switch- ing applications will require a gate trigger pulse of about 30 mA at 2.5 V to control a current of up to 5 A.
11.11 Light emitting diodes
Light emitting diodes (LED) can be used as general- purpose indicators and, compared with conventional fil- ament lamps, operate from significantly smaller voltages and currents. LEDs are also very much more reliable than filament lamps. Most LEDs will provide a reasonable level of light output when a forward current of between 5 mA and 20 mA is applied.
Light emitting diodes are available in various formats with the round types being most popular. Round LEDs are commonly available in the 3 mm and 5 mm (0.2 inch) diameter plastic packages and also in a 5 mm×2 mm rect- angular format. The viewing angle for round LEDs tends to be in the region of 20◦to 40◦, whereas for rectangular types this is increased to around 100◦. The peak wave- length of emission depends on the type of semiconductor employed but usually lies in the range 630 to 690 nm. The symbol for an LED is shown in Figure 11.14(f).
11.12 Varactor diodes
It was shown earlier that when a diode is operated in the reverse biased condition, the width of the depletion region increases as the applied voltage increases. Varying the width of the depletion region is equivalent to vary- ing the plate separation of a very small capacitor such that the relationship between junction capacitance and applied reverse voltage will look something like that shown in Fig- ure 11.18. The typical variation of capacitance provided by a varactor is from about 50 pF to 10 pF as the reverse voltage is increased from 2 V to 20 V. The symbol for a varactor diode is shown in Figure 11.14(h).
0 −2 −4 −6 −8 −10
80 40 20 10 5
Reverse voltage (V) Capacitance (pF)
Figure 11.18
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112 Electrical Circuit Theory and Technology 11.13 Schottky diodes
The conventional p-n junction diode explained in Sec- tion 11.4 operates well as a rectifier and switching device at relatively low frequencies (i.e. 50 Hz to 400 Hz) but its performance as a rectifier becomes seriously impaired at high frequencies due to the presence of stored charge carriers in the junction. These have the effect of momentar- ily allowing current to flow in the reverse direction when reverse voltage is applied. This problem becomes increas- ingly more problematic as the frequency of the a.c. supply is increased and the periodic time of the applied voltage becomes smaller.
To avoid these problems a diode that uses a metal- semiconductor contact rather than a p-n junction (see Figure 11.19) is employed. When compared with con- ventional silicon junction diodes, these Schottky diodes have a lower forward voltage (typically 0.35 V) and a slightly reduced maximum reverse voltage rating (typic- ally 50 V to 200 V). Their main advantage, however, is that they operate with high efficiency in switched-mode power supplies (SMPS) at frequencies of up to 1 MHz.
Schottky diodes are also extensively used in the construc- tion of integrated circuits designed for high-speed digital logic applications.
a k
Anode Cathode
Gold N
(silicon)
Barrier Figure 11.19
Now try the following exercise.
Exercise 37 further problems on semiconductor